Transformers are currently available which include auxiliary windings and sensing coils for sensing the occurrence of a ground fault condition and for providing indication that a fault has occurred. U.S. Pat. No. 3,624,499 discloses the use of transformers having auxiliary sensing equipment for indicating the occurrence of a ground fault condition and for activating a protective relay.
When a fault to ground occurs in a three-phase power system, its presence is signalled by the flow of zero sequence current through the power system and its connected apparatus. Protective relays are available which are capable of distinguishing the presence of the ground fault and triggering corrective action to isolate the affected portion of the system when supplied with a measure of the zero sequence current that is present. One of the most accessible locations for sensing zero sequence current is in a delta-connected winding of a three-phase power transformer where a major component of the current circulates. The usual practice is to position an iron core current transformer so that its core surrounds the delta-connected winding phase lead. A measure of the zero sequence current can be taken from the current transformer secondary winding. An alternative location for sensing the flow of zero sequence current is in a grounded neutral connection of a wye-connected transformer winding, if one exists.
Autotransformers are commonly used in power systems, for system voltages of 115 kV and above. In autotransformers the common ground point connection for the two principal windings is not a suitable measuring point of zero sequence current, since ground fault currents from both primary and secondary voltage systems will flow through the same point. In most such situations a delta-connected tertiary winding is usually available to provide the zero sequence current measurement source.
When autotransformers are used in high voltage substations and are constructed on a three-legged core, the transformer delta-connected tertiary winding can be eliminated for cost saving purposes. Since the tertiary winding is usually employed for providing a source of polarizing (zero sequence) current for ground fault protection relays, some means must be employed for providing the necessary zero sequence current.
The purpose of this invention is to provide a means for sensing zero sequence current in autotransformers not containing a tertiary winding for the purpose of polarizing transmission line protective relays upon the occurrence of a ground fault.